change color of specific residue about ngl HOT 2 OPEN

Also, in this example, if you change the range to (18-20) or make it smaller, it doesn't affect the structure.

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In both examples, 2 different representations are created and are superposed.
If what you are looking for is having a part of a cartoon representation in one color and a selection in another color, you should use only one representation and a custom color scheme. See the documentation here for selection based coloring: https://nglviewer.org/ngl/api/manual/coloring.html#selection-based-coloring

Regarding cartoon representations, there is a known limitation that only stretches of 4 residues or more can be represented. This is a limitation from the implementation of the spline algorithm which requires 4 coordinates for the Catmull-Rom interpolation. For this reason sele: ':B and (10-12)' will probably not render anything.

In the codepen example, I guess that what you are trying to achieve is representing the bulk of the protein structure as a transparent cartoon and only the stretch of interest as opaque colored segment. In that case, you would indeed need to use 2 representations because opacity is a global setting. And then you unfortunately have to workaround the 4 residues limitation.

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